1. Field of the Invention
The present invention generally relates to a machining apparatus for making scroll parts, that is, an article of manufacture having an involute surface or wall such as generally used in a scroll compressor and, more particularly, to a numerically controlled machine tool.
2. Description of the Prior Art
Some recent models of air-conditioner employ a positive displacement scroll pump as a compressor. The positive displacement scroll pump is known as a fluid pump having an impeller or rotor that undergoes an orbital or rotary motion to transport a fluid medium from the inlet to the outlet. The positive displacement scroll pump of the type referred to above is provided with a pair of discs each having a scroll wall of a predetermined height, the discs of the pair being combined together in 180.degree. phase shifted relationship with each other about the axis of rotation with the respective scroll walls contacting with each other. In operation, the discs are driven relative to each other to allow the fluid medium to be guided along the spiral path from the inlet towards the outlet.
The positive displacement scroll pump has numerous merits; smooth in operation, low-noise and low-vibration characteristics and high speed operating characteristics.
The shape of the scroll wall used in the positive displacement scroll pump is represented by an involute curve to meet the geometrical completeness.
The machining of the scroll wall is generally accomplished by converting the mathematic equations descriptive of an intended or desired involute curve into a programmed series of X-Y coordinate values and inputting the programmed series of X-Y coordinate values to a numerically controlled profiling machine or NC machine to cause the tool and the workpiece to undergo a relative movement. The conventional NC machining system however has some problems in that the machining time is limited because of the information processing capability of the NC machining system and in that, in the event that interpolation is carried out in the X-Y coordinate system, an error in movement of the machining tool at a central portion of the workpiece tends to be increased due to a time-lag of the servo follow-up of the drive motor and, therefore, a high speed feed cannot be attained.
In view of the problems inherent in the conventional NC machining system, for example, the Japanese Laid-open Patent Publication No. 62-88507 published Apr. 23, 1987, discloses a NC machining system wherein the center of the machining tool is moved on the straight line tangential to the base circle generating the involute curve while, in synchronism with the movement of the machining tool, the workpiece is rotated about the center of such base circle. According to this prior art machining system, the direction of movement of the machining tool is normal to the involute curve and, therefore, a high speed, highly accurate machining can be accomplished.
However, the prior art NC machining system referred to above requires the machining tool to be offset on the base circle of the involute curve (or on the straight line tangential to such base circle) before the machining is carried out by moving the workpiece in a direction perpendicular thereto. In other words, the prior art NC machining system has to be so designed that the machining tool be supported not only for movement in two direction in a common plane while the workpiece can be rotatably supported, but also for movement in a direction parallel to the machining direction and perpendicular to the common plane in which the machining tool can move in the two directions, that is, in a direction parallel to the axis of rotation of the workpiece.
Therefore, in the prior art NC machining system, the machine requires an increased number of operating axes rendering it to be bulky and costly to make.